With the increasing shortage of bauxite resources, more and more attention has been paid to the use of industrial solid wastes such as fly ash and coal gangue for extracting alumina. Extraction of alumina from high-alumina fly ash and other aluminum-containing resources has drawn increasing attention. For example, minerals like boehmite and kaolinite are rich in Late Paleozoic coal bed and dirt band in Ordos Basin, the content of alumina in the fly ash generated after burning is up to 50%, which is equivalent to the content of alumina in medium-grade bauxite, so this fly ash is an extremely valuable raw material for alumina production. According to statistics, the potential reserve of high-alumina fly ash in the central and western regions of Inner Mongolia is as high as 15 billion tons. Comprehensive development and utilization of aluminum and silicon elements in these high alumina fly ash resources not only guarantees the strategic security of aluminum industry in China, but also is conducive to the protection and development of local environment, therefore, this is a cycling economy industry with great strategic significance.
Many enterprises and scientific research institutes have made considerable tests and researches on how to extract alumina from fly ash, and the process routes are generally classified into two categories, that is, acid method and alkaline method. Although extraction of alumina by means of acid method is low in slag formation and small in equipment investment, it brings quite severe corrosion to equipment, so large-scale production is seldom feasible. The alkaline method, which is further divided into lime sintering method and soda lime sintering method, is a relatively mature technical route for extracting alumina from fly ash and also potentially suitable for industrial popularization. Extraction of alumina from fly ash by means of lime stone sintering method is adopted by Inner Mongolia Melic Sea High-Tech Group Company, the sintering temperature for clinker is 1340-1390° C., and about 9 tons of slag is generated to produce 1 ton of alumina, so there are many disadvantages such as large energy consumption, high cost and large amount of residual slag. The idea of soda lime sintering method originates from the production of alumina from bauxite with low alumina silica ratio by means of soda lime sintering method, and the higher content of alumina in fly ash, the better suitability of this method. However, the alumina silica ratio of fly ash is low, so direct use of soda lime sintering method leads to not only difficult control for clinker sintering due to narrow sintering range, but also large material flow, energy consumption and slag formation. Hence, more attention has been paid to the idea of firstly extracting silicon and secondly extracting aluminum recently. Extraction of partial silica in fly ash at first realizes full utilization of silica source, reduces the amount of slag formation, raises the alumina silica ratio of residues and lowers the difficulty in clinker sintering.
In the patent “method for firstly extracting silicon and secondly extracting aluminum from fly ash” with the patent number 200710062534.7, sodium hydroxide solution with the concentration more than 40% is used for leaching fly ash, silicon is dissolved out in the form of sodium silicate, and then the sodium silicate is separated from alkaline leached slag by a water-added dilution procedure. Na2CO3 solution, which is generated after white carbon black is prepared from sodium silicate solution by carbonization decomposition method, is causticized by CaO, the resultant sodium hydroxide solution is recycled, and clinker is sintered from alkaline leached slag by means of soda lime sintering method or lime stone sintering method in order to produce alumina. In the patent “method for extracting silicon dioxide, alumina and gallium oxide from high-alumina fly ash” with the patent number 200710065366.7, sodium hydroxide solution is also firstly used for leaching silicon in fly ash, white carbon black is prepared from the extracted silicon by carbonization decomposition method, sodium hydroxide is recycled, and alumina is produced from desiliconized fly ash by means of soda lime sintering method. In these patents, the idea of firstly extracting silicon and secondly extracting aluminum based upon the consideration of phase features of fly ash results in the reduction of energy consumption and the improvement of element utilization, however, there are a few disadvantages in white carbon black generated after silicon extraction at present, such as relatively small domestic market share and limited amount in use; furthermore, compared with the production of white carbon black by means of traditional acid method, the production of white carbon black by means of carbonization decomposition method is hardly controllable in technique, more complex in equipment and higher in cost.